In the case of implanting an anchor bolt in a drilling object, such as concrete, there is a method of using an anchor bolt whose base portion is fitted with a slitted cylindrical body provided with a slit formed in its axial direction. In this method, a hole with a predetermined depth is drilled in the drilling object, and the base portion of the anchor bolt is inserted in the drilled hole. Thereafter, the slitted cylindrical body is expanded inside the hole such that frictional resistance is exerted between the cylindrical body and the wall of the hole, and thereby the anchor bolt is implanted.
In this case, the pull-out strength of the anchor bolt implanted in the drilling object greatly depends on the strength of the joint between the wall of the drilled hole and the expanded slitted cylindrical body. Accordingly, an expanded diameter hole portion where the diameter of the drilled hole is expanded by cutting the wall of the hole may be formed in advance in the inner part of the drilled hole at a position corresponding to the base portion of the implanted anchor bolt. In this manner, the pull-out strength of the anchor bolt can be greatly increased.
Therefore, various conventional hole-drilling devices for drilling a hole with an expanded diameter hole portion have been proposed. Such a conventional hole-drilling device drills a straight hole with a predetermined depth in a drilling object, such as concrete, by using a hole-drilling tool such as a hammer drill, and also, forms an expanded diameter hole portion in the inner part of the drilled hole by cutting.
As a hole-drilling device of this type, the inventor of the present invention has previously proposed a hole-drilling device for drilling a hole with an expanded diameter hole portion, which is configured as follows: a drill bit body is fixed to the center of the lower end of a base including a shank; an expanded diameter hole portion forming member is movably attached to a guide groove formed in the side surface of the drill bit body; a cylindrical sleeve is provided on the lower part of the base such that the cylindrical sleeve is movable upward and downward; a lifting spring is provided between the cylindrical sleeve and the expanded diameter hole portion forming member; a pushing plate that moves upward and downward in conjunction with the expanded diameter hole portion forming member is disposed in the cylindrical sleeve; and an application force transmission mechanism is disposed between the cylindrical sleeve and the pushing plate (see Patent Literature 1, for example).
According to the hole-drilling device disclosed in Patent Literature 1, in the case of drilling a hole, during the drilling, when a contacting portion of the cylindrical sleeve comes into contact with the surface of the drilling object and thereby the cylindrical sleeve stops moving downward, movement in which the base still moving downward together with the drill bit body comes closer to the cylindrical sleeve is converted into movement in which the application force transmission mechanism pushes down the expanded diameter hole portion forming member via the pushing plate, such that a cutting bit provided at the lower end of the expanded diameter hole portion forming member (i.e., an undercut bit) is caused to protrude laterally from the drill bit body from an inclined surface formed at the lower end of the guide groove of the drill bit body, and thereby an expanded diameter hole portion is formed in the inner part of the drilled hole by cutting.